1. Field of the Invention
This invention relates to fluid compressors. More particularly this invention relates to an oil free air compressor that is disposed in a controlled atmosphere, wherein the quantity of air leaking therefrom into the controlled atmosphere is minimized.
2. Description of the Prior Art
In cargo containers used for transporting food and other perishables it is necessary to maintain an oxygen poor atmosphere in order to prevent food spoilage, and to prevent damage from insects and similar vermin. Limiting the amount of oxygen in the container, while maintaining the internal temperature within a desired cool range inhibits metabolic processes within the food, and can eventually kill organisms that would attack valuable food cargo. In such containers the need to maximize payload volume limits the space that is available for refrigeration and environmental control equipment, including the system for maintaining the controlled internal atmosphere. This system typically includes an air compressor, heater, and filter that feeds a gas separator that discharges oxygen poor effluent into the chamber, and returns the remainder into the ambient. The gas separator is typically a semipermeable membrane. In order to conserve space, and to protect the system from a harsh marine environment it is standard practice to dispose its air compressor inside the container, within the controlled atmosphere. The protection from salt water and particulates afforded by internal placement of the compressor, and the cool internal temperatures of the operating environment extend the useful life of the air compressor and other components of the system.
The air compressor typically takes in ambient air. Therefore any leakage therefrom will detrimentally affect the controlled atmosphere, and is to be avoided. In the reciprocating compressors in common use today, such leakage can occur from blow-by across the piston rings, and can also occur through a poorly sealed head gasket. Blow-by occurs during upstroke of the piston, and pressurized air escapes from the cylinder's working chamber across the piston rings into the crankcase. The crankcase of a conventional reciprocating compressor is ventilated for cooling, and the blow-by can thus escape into the immediate environment of the compressor. Conventional compressors may also include a fan assembly disposed on an extension of the motor shaft to further cool the unit. The fan increases the space required for placement of the unit.
In Olson, U.S. Pat. No. 5,244,363, it is proposed to limit blow-by in an oilless air compressor by providing a port in the cylinder such that the port is above a first piston ring when the cylinder is positioned at top dead center, and is below a second piston ring when the piston is positioned at bottom dead center. In this arrangement any gas that blows by the first ring exits the cylinder through the port, rather than blowing by the second ring into the crankcase. The fluid can then be exhausted or recycled through the compressor inlet. Other approaches to limiting blow-by involve increasing the number of piston rings. This is expensive and requiring machining special pistons. This approach also increases the complexity of the unit.